Knock sensor

ABSTRACT

A knock sensor for which a fabrication process is simple and moreover which can detect up to a high-frequency region is provided. A sensing element 11 composed of a semiconductor the weight (mass) thereof being 1 g or less and a signal processing circuit 11 are mounted on a fixing pedestal 9. The fixing pedestal 9 is fixed to a connector 2 side by means of adhesive or the like. Additionally, the connector 2 is fixed by means of caulking 16 of a housing 1. As a result thereof, the sensing element 11 is disposed within a space formed by the fixing pedestal 9 and the housing 1.

This application is a continuation-in-part of earlier application Ser.No. 08/198,052 filed Feb. 18, 1994, now U.S. Pat. No. 5,507,182.

TECHNICAL FIELD

The present invention relates to a knock sensor to detect abnormalvibration due to a knocking phenomenon in an engine for a vehicle or thelike.

BACKGROUND ART

A knock control system intended to control a knocking phenomenon,increase engine torque, and improve fuel consumption by sensing aknocking phenomenon in an engine of a vehicle or the like, conveying thepresence or absence of the knocking phenomenon to an engine control unit(ECU), and controlling the ignition timing of spark plugs withincylinders of the engine with the ECU is known conventionally. This knocksensor senses vibration characteristic to the knocking phenomenon, andthe vibration detector thereof has conventionally used a piezoelectricelement composed of ceramic.

Broadly speaking, there are two types of detection methods for knockvibration of this knock sensor. One is a resonance type which causes apiezoelectric element to resonate together with a knocking frequency anddetects output due to the resonance thereof as a knock signal, as isdescribed in Japanese Patent Application Laid-open No. 62-96823 PatentGazette, Japanese Patent Application Laid-open No. 59-164921 PatentGazette, or Japanese Utility Model Application Laid-open No. 62-128332.The other is a flat type which detects a knock signal in a flat regionin which an output signal output by a piezoelectric element is notsubject to the influence of resonance, as described in Japanese UtilityModel Application Laid-open No. 57-99133. Because the former causesresonance with the knock vibration, output with a good signal-to-noiseratio is obtained, but conversely only a unique knock vibration can bedetected, and in the case of an engine with many cylinders it isimpossible to detect knock vibration of all cylinders at a singlelocation, and the problem exists that a plurality of knock sensors arerequired. On the other hand, the latter can detect knock vibration ofvarious frequencies, but the possibility exists that, aside from theinfluence of the resonant frequency of the element itself, the vibrationof other components may exert an influence on the vibration detectionregion, and there exists the problem wherein the degree of freedom indesign of the knock sensor itself is narrow.

As the structure of these sensors, a structure disposing a piezoelectricelement composed of a vibration detector within a space formed by ahousing made of metal (or a housing composed of a strong material to besubstituted thereby) having a projection of screw configuration so as tobe installed directly on an engine and of a connector molded of resinwhich allows connector connection with an external portion is common.

Accordingly, there are two types of piezoelectric element installations:a type firmly fixed to the housing side by means of a screw or the like,and a type fixed in a state fixed to a stem of metal which becomes afixing pedestal (or a strong fixing pedestal to be substituted thereby)on the connector side. Additionally, for the latter there exist, assimilar types thereof, a type fixed to the stem, and not fixed directlyto the connector side but fixed by means of caulking, and a type whereinthe contact point of the step and housing are connected by means ofgluing or welding or the like. That is to say, the latter can be termeda type fixed to the stem and disposed within a spaced formed by the stemand housing.

In a case where the piezoelectric element is fixed to the housing side,resonant frequency is high because the housing itself is made of metal,the housing itself does not resonate due to engine vibration, andinfluence thereof is not exerted on the piezoelectric element. However,it is necessary to perform the electrical connection from thepiezoelectric element to the connector terminal by means of for examplelead wires, the connector and housing must be fixed by means of caulkingor the like to fix the piezoelectric element to the housing and connectthe lead wires, and there exists the problem of a difficult fabricationprocess.

On the other hand, in a case of fixing to the connector side, becauseelectrical connection from the piezoelectric elements to the connectorterminal can be performed at the connector side, it is sufficient to fixa connector whereon a piezoelectric element is fixed to the housing bymeans of caulking or the like, and the fabrication process becomessimple. However, because the connector is generally made of resin, theresonant frequency is low, and in a case whereby the piezoelectricelement is connected to the connector with nothing, resonance of theconnector is conveyed without being attenuated by the piezoelectricelement, and there exists a problem of influence being exerted on signaldetection. To prevent this, mounting on a stem made of metal (or astrong material the Young's modulus of which is not less than metal) soas to impede vibration conveyance of the connector is required.

In this case, with a piezoelectric element of resonant type thedetection signal is the resonance output of the piezoelectric element,and so there is no influence if output due to resonance of the connectoror the like is to a certain extent smaller than that due to resonance ofthe piezoelectric element, but with a piezoelectric element of flattype, in a case whereby frequency thereof is a flat region, thesignal-to-noise ratio may be caused to decline greatly, and so vibrationfrom the connector must reliably be impeded by making the thickness ofthe stem thicker. This can be said to be the same also for the typeconnecting the contact point of the stem and housing by means of gluingor welding or the like. That is to say, with partial welding or the likeadequate suppression of resonance of connector vibration cannot beperformed.

Accordingly, this problem is subjected to the most influence by theweight of the piezoelectric element itself, and in a case of identicalstem thickness, if the weight thereof becomes heavier the resonantfrequency of the stem declines, and the influence thereof appears evenmore strongly. For this reason, it is necessary to cause the weight ofthe piezoelectric element to be reduced, but if this is done a problemof a drop in sensitivity appeared. Consequently, in order to avoid theinfluence of a decline in resonant frequency while maintainingsensitivity, the detection frequency region which assumes flatcharacteristics becomes a maximum of approximately 10 kHz, and theproblem exists wherein detection up to a high-frequency region is notpossible.

Accordingly, in view of this problem it is an object of the present ideato provide a knock sensor having a vibration detector of flat typecapable of detecting a plurality of knock signals and fixed on a fixingpedestal as well as disposing the vibration detector within a spaceformed by the fixing pedestal and a housing, having a simple fabricationprocess and moreover capable of detection up to a high-frequency regionwithout causing required sensitivity to decline.

The present inventors firstly made verification regarding the vibrationdetector. As a result of investigation by the present inventors, it wasunderstood that in a case whereby the detection region of the knocksignal is taken to be a maximum of approximately 15 kHz, the resonantfrequency of a stem (made of metal) which does not overly influence thisdetection region becomes a minimum of approximately 40 kHz. When a stemthickness whereby resonant frequency becomes 40 kHz was investigated, itwas understood that roughly 2.7 mm was required, as shown in FIG. 20.This is a simulation performed by means of the model indicated in FIG.22, and this stem 30 takes the diameter thereof to be 19 mm, a regioncombining the vibration detector and other circuitry thereof is taken tobe a load region 31, and the diameter of the surface on which the loadregion 31 is mounted is taken to be 16.5 mm. Additionally, this stem 30has a step, and a weld portion M welded to a housing (not illustrated)is formed on the step surface thereof so as to approach actual stemconfiguration. As can be understood from this drawing as well, if stemthickness D is caused to change without changing the diameter, it isunderstood that the resonant frequency of the stem rises. This can beunderstood if it is considered that, wherein the weld portion M isfixed, there exists an image whereby a solid configuration is moredifficult to vibrate than a plate configuration.

Additionally, FIG. 21 indicates change in resonant frequency of the stemin cases whereby stem thickness are taken to be 2.8 mm and 3.5 mm andfurther in a case whereby the load mounted on these stems is caused tochange. FIG. 21 is data in a case whereby the load region 31 is causedto change from 0.1 g to 4.6 g. It is understood from this drawing thatresonant frequency declines in a case whereby a load is applied to thestem. Consequently, even in a case whereby a load is applied increasedthickness is required in order to maintain the resonant frequency atapproximately 40 kHz. In a case whereby for example the load is taken tobe 4.6 g, in order to make the resonant frequency of the stem to be 40kHz, when in FIG. 21 the stem thickness is 2.8 mm the resonant frequencythereof is 20 kHz, and consequently it is necessary to double theresonant frequency. Accordingly, if in FIG. 20 the resonant frequencyand stem thickness are taken to be in a linear relationship, a stemthickness of 4.4 mm becomes necessary.

In actuality, the weight of a piezoelectric element is approximately 20g, and in order to cause the resonant frequency of a stem mounted withthis piezoelectric element to be 40 kHz, if resonant frequency becomes10 kHz when hypothetically the stem thickness is 2.8 mm in a casewhereby load is taken to be 20 g, the stem thickness must be increasedby 3 mm even at a low estimate to approximately 5.8 min.

Consequently, there is not only enlargement as a knock sensor, but in acase whereby a through-hole for the purpose of passing a pin to connectto the connector terminal is made in the stem by punching or cutting,when the strength of the punch pin or cutting drill is considered, it isnecessary to make the diameter thereof to be approximately identical tothe stem thickness, and it is necessary to form a considerably largethrough-hole on the stem. In a case of the foregoing stem, if avibration detector of a piezoelectric element or the like is taken to bemounted on the surface on which load is applied, it is necessary to makea 5.8 mm through-hole on a surface with a diameter of 16.5 mm, and themounting region of the element is constrained. Consequently, if mountingof signal processing circuitry other than the piezoelectric element isto be attempted, the diameter of the stem must be enlarged. However,enlargement of the diameter of the stem signifies a decline in resonantfrequency even if thickness is the same, and stem thickness must beincreased further in order to maintain the resonant frequency at thesame value. If this occurs, the size of the through-hole must also beenlarged proportionately to the stem thickness as described above,repeating a vicious cycle and generating a failure in which designvalues are not obtained. Consequently, in a case whereby a piezoelectricelement is employed in a vibration detector thereof, the limit for themaximum detection frequency that can be obtained is 10 kHz. In addition,even hypothetically if designed with a large through-hole, the processbecomes complex because of the device needed to make the through-hole.

Consequently, in order to enable detection up to high frequencies, it isnecessary to use an article lighter than a piezoelectric element. Asshown in FIG. 21, even when stem thickness is approximately 3.5 mm, aload allowing the resonant frequency of the stem to be established at 40kHz is roughly 1 g. If stem thickness is approximately 3.5 mm, formationof a through-hole also becomes possible without major change.

Additionally, even if the fixing pedestal is not a metal stem,ultimately the resonant frequency resonant frequency of the fixingpedestal will undoubtedly decline and exert an influence on vibrationdetection.

Accordingly, the present inventors gave attention to a semiconductoracceleration sensor formed on a semiconductor substrate used in anairbag and the like as allowing the vibration detector to be made to be1 g or less. This is structured from a weight (mass), a beam supportingthis weight (mass), and a frame to which the beam is fixed by means ofetching or the like on for example a semiconductor silicon substrate.The weight (mass) vibrates by means of vibration of an external portion,and vibration is detected by sensing stress generated in the beam bymeans of vibration thereof; the inventor as well has shipped multipleexamples to market. Accordingly, a vibration detector according to thissemiconductor is extremely compact with good sensitivity, and thicknessthereof can adequately be made to be 1 g or less.

SUMMARY OF THE INVENTION

Consequently, a knock sensor according to the present inventioncomprises a sensing element wherein a weight (mass) and a beam tosupport the weight mass as well as to sense vibration are formed on asemiconductor substrate, a fixing pedestal to fix the sensing elementand also the strength of which is as hard as metal, a connector portiondisposed on a surface side of the fixing pedestal opposite that whereonthe sensing element is installed and also conveying output of thesensing element to an external portion, and a housing disposed so as tocover the sensing element as well as being installed on an engine.

According to the foregoing structure, in the present invention a sensingelement formed on a semiconductor substrate is taken as a vibrationdetector, and so the weight thereof can be reduced to an extreme degree.Consequently, as a knock sensor disposing the vibration detector withina space formed by the fixing pedestal and the housing, a knock sensorcapable of detection up to a high-frequency region without causingrequired sensitivity to decline and moreover capable of detecting aplurality of knock signals can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a knock sensor showing a first embodiment.FIG. 2 is a sectional view of a knock sensor showing a secondembodiment. FIGS. 3(a) and 3(b) are characteristic diagrams for thepurpose of indicating effects of the second embodiment. FIG. 4 is asectional view of a knock sensor showing a third embodiment. FIG. 5 is ablock diagram indicating a signal processing circuit. FIG. 6(a) is agraph showing frequency characteristics of a knock sensor. FIG. 6(b) isa graph showing a relationship between beam width, resonant frequency,and sensitivity. FIG. 7(a) is a view indicating a vibration detector ofcantilever supported beam structure. FIG. 7(b) is a view indicating avibration detector of doubly supported beam structure. FIG. 8(a) is agraph showing a relationship between sensitivity and resonant frequencyof a vibration detector due to length of a weight (mass) of cantileverbeam structure. FIG. 8(b) is a graph showing a relationship betweensensitivity and resonant frequency of a vibration detector due to lengthof a weight (mass) of doubly supported beam structure. FIG. 9(a), FIG.9(b), FIG. 10(a), FIG. 10(b), FIG. 11(a), and FIG. 11(b) are graphs eachshowing a relationship between beam thickness, resonant frequency, andsensitivity in a case whereby respective beam lengths are fixed atcertain values in a vibration detector of doubly supported beamstructure, wherein: FIG. 9(a) is when beam length is taken to be 0.05mm; FIG. 9(b) is when beam length is taken to be 0.10 mm; FIG. 10(a) iswhen beam length is taken to be 0.20 mm; FIG. 10(b) is when beam lengthis taken to be 0.25 mm; FIG. 11(a) is when beam length is taken to be0.30 mm; and FIG. 11(b) is when beam length is taken to be 0.35 mm. FIG.12 is a graph showing a formative region of beam width and beamthickness. FIG. 13 is a graph showing a formative region of beam widthand beam thickness in a case whereby resonant frequency in FIG. 12 istaken to be 60 kHz. FIG. 14(a) and FIG. 14(b) are graphs each showing aformative region of beam width and beam thickness in a case whereby aweight (mass) has been enlarged. FIG. 15(a) is a structural plane viewof a sensing element employed in the foregoing embodiment. FIG. 15(b) isa sectional view of FIG. 15(a). FIG. 16 is a Wheatstone bridge circuitdiagram. FIG. 17 is a sectional view indicating a sensing element in astate fixed on a substrate. FIG. 18(a) is an enlarged view indicating aportion of a sensing element. FIG. 18(b) is a sectional view of FIG.18(a). FIG. 19 is a view showing a state mounted with an element fixedon a fixing pedestal. FIG. 20 is a graph showing a relationship betweenstem thickness and resonant frequency. FIG. 21 is a graph showing arelationship between load region and resonant frequency. FIG. 22 is aview showing a model obtaining the characteristic diagrams of FIG. 20and FIG. 21.

BEST MODE FOR CARRYING OUT THE INVENTION

A first embodiment of a knock sensor according to the present inventionwill be described hereinafter with reference to the drawings.

A first embodiment of knock sensor structure will be described first.

FIG. 1 is a sectional view of a knock sensor. A substrate 9 is glued toa connector 2 formed integrally with a terminal 6 by means of adhesive13. A fixing pedestal 9 is composed of a ceramic substrate which isstronger than metal, and a capacitor (layer capacitor) and filter 8forming an EMI filter are built-in or surface-mounted within this.Additionally, a sensing element 11 composed of a semiconductor whichwill be described below and a signal processor 10 composed of a powersupply circuit, amplifier circuit, and knock signal discriminatorcircuit are glued to the fixing pedestal 9, with electrical conductancebetween elements or with an external portion provided by a wire 14. Inaddition, the fixing pedestal 9 and connector 2 are electrically bondedby means of a socket 7a, 7b established on the fixing pedestal 9.Accordingly, a housing 1 composed of metal is fixed to the connector 2by means of caulking 16 and adhesive 4 so as to cover the fixingpedestal 9. Additionally, the fixing pedestal 9 is sealed airtightly bymeans of an O-ring 3 and adhesive 5. This knock sensor is fixed to anengine for detection of knocking phenomenon by means of a screw 15 ofthe housing 1.

In order to enable the combined weight of the sensing element 11 to bedescribed below and signal processor circuit and the like to be made tobe approximately 0.1 g, a knock sensor structured in the foregoingmanner can maintain the resonant frequency of the substrate atapproximately 40 kHz even when connected to the connector side via thefixing pedestal 9. Consequently, even if the sensing element isinstalled on the connector side via the fixing pedestal, a sensorcapable of detecting up to a high-frequency region of approximately 15kHz or more can be provided. Additionally, in comparison with a casewhereby a sensing element is installed on the housing side, according tothe present structure the fixing pedestal substrate on which the sensingelement is priorly mounted can be installed on the connector side, andso the fabrication process is simplified and connection of the connectorterminal and substrate connected done easily and reliably.

However, in a knock sensor according to the present embodiment, themaximum frequency to detect is roughly 15 kHz or more which is high, andif the resonant frequency of the connector is considered, there existslimitations in structure which satisfy performance. According to thepresent structure, compactness and light weight of the mounted deviceare attempted by employing a sensing element composed of a semiconductorfor the vibration detector instead of employing a conventionalpiezoelectric element, and along with this, the foregoing structure wasachieved by optimizing the composition and configuration of theconnector as well as Young's modulus of the adhesive fixing theconnector and housing. This optimization was analyzed by means of FEM.As a result of this, the Young's modulus of the composition of theconnector was 1,000 kgf/mm² to 2,000 kgf/mm², the adhesive portionconnector thickness d indicated in FIG. 1 was 1 to 3 mm, and theadhesive Young's modulus was 10 to 2,000 kgf/mm², by means of which thepresent structure was established.

Next, FIG. 2 indicates a modification of the first embodiment as asecond embodiment. This is fixed to the connector 2 by means of adhesiveor the like on the entirety of the rear surface of the fixing pedestal9. Accordingly, a connector terminal 6a is connected to the sensingelement 11 or a signal processing circuit 10 or the like by means of awire bond 14. The signal processing circuit 11 and sensing element 10according to the present embodiment are sealed by means of a can 24 andsimultaneously the can 24 is caused to contact silicone gel 25injection-hardened within the housing 1. By doing this, vibration of theconnector 2 conveyed via the fixing pedestal 9 can be absorbed by thesilicone gel 25 via the can 24, and resonance of the fixing pedestal 9can be suppressed. Consequently, resonance of the connector is notconveyed directly to the sensing element. Accordingly, the degree ofdesign freedom of the vibration countermeasures thereof is increased incomparison with the first embodiment.

Vibration suppression of the fixing pedestal 9 by means of the siliconegel 25 is indicated in FIGS. 3(a) and 3(b). FIG. 3(a) shows a state withno silicone gel, and FIG. 3(b) shows a state with silicone gel. In thefixing pedestal 9 of the sensor with no silicone gel there isconsiderable fabrication at the frequency bands of 20 kHz, 30 kHz, and50 kHz, but it is understood that in the fixing pedestal 9 of the sensorwith silicone gel, vibration is suppressed at the foregoing singularitypoints. Moreover, the material to absorb vibration is not exclusivelysilicone gel in particular. Additionally, it is also acceptable for theterminal 6a to be a structure penetrating the fixing pedestal 9.

Next, as a third embodiment, FIG. 4 indicates a device employinghermetic seal technology as a method to seal a sensing element composedof a semiconductor.

In this case, a sensing element 11 and signal processor 10 are fixed viaa substrate 17 to a metal stem 9', which becomes a fixing pedestal, andmoreover are fixed by welding to a housing 1. The structure of the metalstem 9' at this time has a thickness of 2.8 mm and a diameter of 19 mm,as shown in FIG. 21, and the diameter of the surface whereon the sensingelement and so on are mounted is 16.5 mm, and the diameter of thethrough-hole passed through by an extraction pin 18 is approximately 2.8mm. According to this embodiment, the sensing element 11 and signalprocessor 10 are mounted on the metal stem 9' via the substrate 17, butthe overall weight of the mounted components becomes roughly 0.3 g.Additionally, as will be described later it is acceptable for thissubstrate 17 to be absent as shown in FIG. 19.

In this manner, according to the present embodiment a vibration detectorcomposed of a semiconductor is employed as the sensing element,similarly to the first embodiment, and so sensitivity is favorable withcompactness, the weight thereof can be made to be an extremely lightapproximately 0.3 g, and even when mounted on a thin metal stem in theabove-described manner, the resonant frequency of the metal stem can becaused to be 40 kHz with substantially no decline. Additionally, thethickness of the metal stem is approximately 2.8 mm, and thethrough-hole through which the extraction pin passes becomes easilyformable. Consequently, a knock sensor of a structure whereby thefabrication process is simplified, and furthermore which can detect upto high-frequency regions with no drop in sensitivity, can be provided.

Additionally, in comparison with the structure of the first embodimentthere is no particular need to perform vibration countermeasures in thecomposition of the connector or the like. By means of this, theselection range is further expanded for the composition of the connector2 and the composition of the adhesive. In addition, the sensing element11 and signal processor 10 are reliably sealed airtightly by means ofprojection welding and glass sealing of the extraction pin 18.

Moreover, the foregoing third embodiment adopts an airtight-sealstructure which employs hermetic seal technology to fix the extractionpin, but it is also acceptable for example to fix the extraction pin bymeans of filler-containing adhesive or the like instead of hermetic sealglass material, so as to seal airtightly. Alternatively, it is alsoacceptable to adopt an airtightly sealed structure whereby a memberother than glass is inserted into the through-hole.

Furthermore, according to the foregoing first through third embodiments,by means of achieving compactness of the sensing portion to detectvibration, it becomes possible to dispose an amplifier circuit toamplify the output signal of the sensing element and a discriminatorcircuit to process the amplified signal and output a knock signal to anexternal portion on the same substrate without enlarging the diameter ofthe ceramic substrate which becomes the fixing pedestal or of the metalstem. Effects which this yields will be described hereinafter.

Firstly, FIG. 5 indicates a block diagram of a vibration detectioncircuit of the present structure. A signal detected by means of thesensing element 11 is amplified by an amplifier circuit 10a,synchronized to an ignition signal from an engine ECU and determined tobe a knocking phenomenon such as knock signal discriminator circuit 10b,and output to the ECU. Further, 10c is a power supply circuit formed onthe same substrate as the amplifier circuit 10a and knock signaldiscriminator circuit 10b, and supplies electrical power from thebattery of the vehicle or the like to the respective circuits. Aconstant voltage of for example 5 V is caused to be generated as thevoltage thereof. Consequently, in a case whereby voltage of the batteryis taken to be 12 V, the difference thereof is 7 V, and a stable voltagecan be provided to the respective circuits even if battery voltagefluctuates due to noise or the like.

Moreover, as is shown in this drawing, a signal amplified by theamplifier circuit 10a is analog output, but a signal discriminated bythe knock signal discriminator circuit is connected to the ECU asdigital output. Consequently, a structure which is strong with respectto noise can be achieved as the knock sensor.

By means of this, in a case whereby for example a ground (GND) is takenfrom the engine (chassis), with respect to when GND potential issubjected to influence from another circuit and fluctuates during analogoutput and the fluctuation thereof is overlaid on the analog output andis erroneously processed at the next stage of signal processing, withdigital output, even if fluctuation in GND potential is overlaid onoutput, discrimination is made at 1/2 of the rising-edge height in thenext stage of signal processing, and so if the noise is notconsiderable, the influence of the noise is nor received. Consequently,it becomes possible actually to take the GND from the engine via asocket 7b and the housing, as shown in for example FIG. 1. By means ofthis, wiring for GND use becomes unnecessary, and reliability isimproved, and along with this the need to provide a connector terminalfor GND use is eliminated and the connector can be made smaller.

In this manner, a structure with a small and light sensing element ispossible and so it becomes possible to mount other circuit elements onthe fixing pedestal, by means of which a sensor which is strong withrespect to fluctuations of the battery and GND potential fluctuationscan be provided.

Next, a sensing element composed of a semiconductor element employed inthe present embodiment will be described hereinafter.

The present inventors firstly investigated the frequency range whereinthe resonant frequency of the sensing element should be established.Here, as shown in FIG. 6(a), in a case whereby the maximum detectionfrequency was taken to be fs and the resonant frequency was taken to befr, the resonant frequency fr must be established so that the resonantfrequency is not affected, such that the detection region assumes flatcharacteristics. However, in detecting vibration due to an engineknocking phenomenon, the problem arises as to whether the sensingelement composed of a semiconductor which is employed in a semiconductoracceleration sensor can satisfy the basic characteristics of the sensingportion such as sensitivity, resonant frequency, and fracture strength.The weight (mass) configuration and beam configuration, particularly thebeam thickness and beam width, are in a close relationship with resonantfrequency and sensitivity, and if beam thickness and beam width arecaused to be changed, resonant frequency and sensitivity change greatly.If beam width is taken as an example, resonant frequency is proportionalto the square root of beam width, and sensitivity is inverselyproportional to beam width. That is to say, beam width and resonantfrequency can be said to be in a mutually contradictory relationshipwith beam width and sensitivity. This is indicated respectively by thesolid line and the broken line in FIG. 6(b). Consequently, the problemarose as to whether a structure exists which adequately satisfies thebasic characteristics of sensitivity and resonant frequency.

In order to obtain stabilized output, the present inventors firstlyneeded to establish resonant frequency so that sensor output yieldedflat characteristics even at maximum detection frequency. As isunderstood from examination of FIG. 6(a), output increases in the mannerof an exponential function from the flat region to the peak of resonantfrequency. Consequently, resonant frequency must be established so thatthe rising edge of the output due to the peak of resonant frequency doesnot overlap with the maximum detection frequency, such that fluctuationin output at the maximum detection frequency becomes a sufficientlysmall value. However, this rising edge of the output is extremelydifficult to determine by means of theoretical analysis, and does notbecome clear until an element is actually floorboard and measured. Inthis manner, it is not easy to design a semiconductor accelerationsensor which becomes a vibration detector of a knock sensor.

Accordingly, the present inventors took and tabulated data from aconventional acceleration sensor, and as a result, sought out arelationship of resonant frequency fr and maximum detection frequency fswherein the response frequency region of sensor output is constantlyflat as will be shown below.

    f.sub.r ≧A·.sub.s (2.5≦A≦4)

However, A is a constant determined by means of the support method ofthe weight (mass). Accordingly, by means of discovering thisrelationship, it was determined that a knock sensor which can satisfybasic characteristics of vibration detection of sensitivity, resonantfrequency, and fracture strength is realizable even when a semiconductoracceleration sensor is employed as a vibration detector thereof.

Accordingly, it is understood that if maximum detection frequency istaken to be 15 kHz, it is acceptable for the resonant frequency of thesensing element to have a value of approximately 40 kHz or more.

Next, sensor structure was investigated. As shown in FIG. 6(b), resonantfrequency and sensitivity are in a tradeoff relationship in for examplebeam width, and a sensor structure which simultaneously satisfies therequired resonant frequency and sensitivity was investigated. Thepresent inventors investigated the structure indicated in FIGS. 7(a) and7(b). In the device indicated in FIG. 7(a), a weight (mass) 41 issupported by means of a beam 42. Additionally, FIG. 7(b) indicates astructure wherein a weight (mass) 21 is supported by means of four beams22. The slanted-line areas in the drawings are portions removed by meansof etching. Hereinafter, the device indicated in FIG. 7(a) will be takento be a cantilever beam structure and the device indicated in FIG. 7(a)will be taken to be a doubly supported beam structure.

FIGS. 8(a) and 8(b) indicate investigation into whether design solutionstaking resonant frequency to be 40 kHz or more and sensitivity to be 12μV/G or more actually exist for the foregoing cantilever beam structureand doubly supported beam structure. Herein, the horizontal axis istaken to be the length of the weight (mass) 41 or 21, the vertical axisof the left-hand side is taken to be resonant frequency, the verticalaxis of the right-hand side is taken to be sensitivity, resonantfrequency is indicated by means of a solid line, and sensitivity isindicated by means of a dotted line. According to these two drawings, adesign solution for cantilever beam structure exists only in theextremely narrow range wherein the length of the weight (mass) for thepurpose of satisfying the above-described conditions is approximately0.35 to 0.36 mm, and in process it is virtually impossible to align theweight (mass) with this range with good planned yield. In contrast tothis, it was understood that with a device of doubly supported beamstructure, the length of weight (mass) is approximately 0.6 to 2.7 mm,which is a fabricatable range with sufficiently good planned yield.Consequently, a doubly supported beam structure is fabricatable withgood planned yield as a structure which simultaneously satisfies bothsensitivity and strength.

Additionally, the length and thickness of the beam were investigatednext.

Herein, for the beam width WB indicated in FIG. 7(b), the minimum valuebecomes essentially 0.13 mm due to the piezoelectric element resistanceelement formed within the width thereof. Consequently, in a case wherebycompactness of the element is attempted, determination is according tothe size of the weight (mass) or the length of the beam. For the weight(mass), the surface area of the top surface indicated in FIG. 7(b)becomes roughly 0.9 mm² of 1.2×0.7 due to the machining precisionthereof. Additionally, the thickness of the weight (mass) is dependenton the thickness of the machined wafer, and becomes substantially 0.3 mmin the present sample. Consequently, investigation into the length ofthe beam LB indicated in FIG. 7(b) becomes a critical point in terms ofcompactness.

In FIGS. 9(a) and 9(b), FIGS. 10(a) and 10(b), and FIGS. 11(a) and11(b), the formative region of beam width wherein resonant frequency andsensitivity satisfy conditions similar to the foregoing when the lengthof the beam LB is caused to be varied as a parameter from 0.05 to 0.35mm was investigated. As a result of this, as shown in FIG. 12, theformative beam thickness interval and beam length obtained on the basisof the above-described conditions are indicated. Herein, because beamthickness is formed by means of wet etching employing an etching liquid,it is extremely difficult to perform etching with precision on themicron order, and there is fluctuation of roughly 4 μm. Consequently, aregion from which this 4 μm is subtracted becomes a region wherein beamscan be formed with good planned yield. If this is determined accordingto FIG. 12, beam length becomes 0.05 to 0.215 mm. The lower limit of0.05 mm indicates the machining limit.

Herein, if for the vibration detector the maximum detection frequency istaken to be 15 kHz, approximately 40 kHz or more is acceptable, and itis sufficient to design according, as was described above, but becausethe sensing element is a semiconductor and has high crystallinity, Qvalue during resonance is extremely high. Therefore, if resonance of thefixing pedestal and sensing element become identical, the sensingelement causes large vibration when the vibration component of this isadded, and the there is possibility of a drop in the signal-to-noiseratio or even destruction. Accordingly, in order to avoid this, therelationship between the formative beam thickness interval describedabove and beam length was investigated, taking the resonant frequency ofthe sensing element to be 60 kHz or more. The results of this areindicated in FIG. 13. In a case such as this whereby resonant frequencyis taken to be 60 kHz or more and sensitivity is taken to be 12 μV/G ormore, the beam length region wherein the formative beam thickness existsbecomes approximately 0.05 to 0.1 mm.

Additionally, the basis of conditions similar to FIG. 13 was determinedfor a device wherein the size of the weight (mass) is substantiallydoubled together with taking the resonant frequency to be 60 kHz inconsideration of the signal-to-noise ratio and strength as describedabove. This is data obtained employing the weight (mass) in FIG. 14(a)which is 2.02 times larger than the device for which data was obtainedin FIG. 13 and a weight (mass) in FIG. 14(a) which is 2.14 times largerthan the device for which data was obtained in FIG. 13. This is a devicefor which the upper-limit value was investigated, and it is understoodfrom this drawing that the upper-limit value for length of the beambecomes 0.215 mm.

Consequently, in a doubly supported beam structure in a case wherebyreduction of the surface area of the weight (mass) and compactness areattempted and moreover consideration is given to minimum conditions(i.e., resonant frequency of 40 kHz or more and sensitivity of 12 μV/Gor more) as a knock sensor, in a case even of large size wherebyresonant frequency is taken to be 60 kHz or more and the signal-to-noiseratio and strength are considered, beam length becomes 0.05 to 0.215 mm.Furthermore, in a case where size is made compact and thesignal-to-noise ratio and strength are considered, a favorable beamlength of approximately 0.05 to 0.1 mm is yielded.

Additionally, the considerations which will be described hereinafter aremade for a doubly supported beam structure such as that described above.

That is to say, several resonance points exist, not one; primaryresonance whereby the weight (mass) resonates perpendicularly andsecondary resonance whereby the weight (mass) resonates so as to betwisted are in particular large; when the primary resonant frequency andthe secondary resonant frequency are proximate the weight (mass)vibrates complexly, and the beam is destroyed. Consequently, if beamstrength is considered, it is demanded that the primary resonantfrequency and the secondary resonant frequency be separated.

From the foregoing, a structure was adopted wherein a weight (mass) 21of oblong configuration is supported by means of four beams 22 as shownin FIGS. 15(a) and 15(b). According to this structure, generation oftorsion is difficult, and so the primary resonant frequency and thesecondary resonant frequency can be separated. Additionally, because aWheatstone bridge circuit can be formed in vibration detection, highsensitivity is obtained.

According to the present embodiment, resonant frequency is establishedto be 60 kHz or more and sensitivity is established to be 12 μV/G ormore, and the beam structure and weight (mass) structure are establishedas will be described hereinafter. Beam width W_(B) indicated in FIG.15(a) becomes 0.13 mm. In addition, beam length LB becomes 0.11 mm.Additionally, because a piezoelectric element is formed on the beam,beam thickness T_(B) is determined by means of the pn junction thereof,and according to the present embodiment is taken to be 13 μm. Moreover,the thickness of the weight (mass) T_(M) indicated in FIG. 15(b) isdetermined by means of the thickness of the wafer utilized, andaccording to the present embodiment is taken to be 0.3 mm. Furthermore,the width of the weight (mass) W_(M) is determined by means of thethickness of the weight (mass) T_(M) because, due to the sensitivityrelationship, the etching surface orientation for the purpose of formingthe weight (mass) is taken to be a (100) surface and the substrate iscaused to be tapered by means of etching. According to the presentembodiment, a margin is added and this is taken to be 0.7 mm.Accordingly, the length of the weight (mass) L_(M) must be establishedso as to primary Rapid Micro Controller and secondary resonance withthis width of the weight (mass) W_(M) as a basis, and according to thepresent embodiment is taken to be 1.2 mm.

These values may be established as required, taking the resonantfrequency to be established as a basis and giving consideration to thefabrication process, the wafer utilized, the relationship betweenprimary and secondary resonant frequencies, or the like.

A detection method to detect vibration of the weight (mass) will bedescribed next.

According to the present embodiment, piezoelectric elements 23a to 23dare disposed as shown in FIG. 15(a). In this manner, disposition is withelement 23a and element 23c on the weight (mass) side and element 23band element 23d on the fixed frame side, and directivity is improvedwhen Wheatstone bridge wiring is performed as shown in FIG. 16. That isto say, in a case whereby the weight (mass) 21 vibrates perpendicularly,the element 23a (element 23c) and the element 23b (element 23d) aresubjected to mutually differing stresses, i.e., tensile stress andcontraction stress, and sensor sensitivity is improved. Additionally,with respect to torsion vibration (vibration of another axis), theelement 23a (element 23c) and the element 23b (element 23d) aresubjected to mutually identical stresses, and so the other-axissensitivity at the Wheatstone bridge of FIG. 16 can be canceled.Furthermore, FIG. 15(b) is a sectional view taken along a line along thebeam section of FIG. 15(a). Additionally, the m and n of FIG. 16 arestresses, and V represents the power supply. Furthermore, the number ofpiezoelectric elements disposed is not exclusively four, and may be forexample eight.

In the foregoing manner, a resonant frequency of 60 kHz or more andsensitivity of 12 μV/G or more can be satisfied by designing elements.

Additionally, because the detection frequency of approximately 15 kHzhas become considerably high in comparison with several hundred Hz for aconventional acceleration sensor, the beams of the acceleration sensorcan be made strong. By means of this, displacement of the weight (mass)21 in the detection vibration region becomes approximately several μm,and by means of providing this void with the adhesive 18 (thicknessapproximately 10 μm), the concavity (dotted line portion in the drawing)required in the pedestal for the purpose of installing a conventionalacceleration sensor becomes unnecessary. By means of this, the processfor the purpose of forming a concavity in the pedestal 12 can beeliminated. Moreover, the void of the pedestal 12 and weight (mass) 21is the approximately 10 μm thickness of the adhesive 18, which isnarrow, and so in a case whereby the weight (mass) 21 vibrates greatlydue to a strong shock, the pedestal 12 becomes a vibration stopper forthe weight (mass) 21, and destruction of the sensor beams can beprevented.

Additionally, according to the present embodiment a fracture strength of47,000 to 48,000 G is obtained. This fracture strength becomes a problemin particular in the fabrication process and the transfer process andthe like up to installation of the knock sensor, and is a value designedon the basis actual drop testing on concrete and on oak wood, determinedwith consideration for actual drop shock and resonant frequency of thebeams. In a case such as the present of a resonant frequency of 60 kHz,results have been obtained that it is sufficient if there is up to50,000 G, and so the foregoing value of 47,000 to 48,000 G can be termedsubstantially sufficient.

Furthermore, if it is desired that this be raised up to 50,000 G, it isacceptable to use a thin film for the beam mounting base portion 50indicated by dotted lines in FIG. 18(a) and in the sectional view ofFIG. 18(b) taken along line A--A' thereof. When done in this manner, thestress concentration of the portions indicated by circles in the drawingcan be alleviated, and beam strength can be increased.

Additionally, an influence effect which will be indicated hereinafter isalso demonstrated in a case of the structure indicated in FIGS. 18(a)and 18(b).

That is to say, when the weight (mass) has vibrated, it is preferablethat the piezoelectric elements be disposed at the mounting base of theweight (mass) and beams which is one fixing point or the mounting baseof the beams and frame which is another fixing point, where beam stressis greatest. However, when the elements become small as in the foregoingembodiment, extremely high precision is demanded in the positionalalignment of the mask for the purpose of forming the piezoelectricelements. In a case where mask slippage is caused and the piezoelectricelements are shifted from the mounting base of the weight (mass) andbeams or the mounting base of the beams and frame, and piezoelectricelements for formed only in the middle of the weight (mass) or only inthe middle of the frame without being attached to the beams, the stresssensitivity of the elements thereof declines to an extreme degree.Accordingly, by means of adopting a structure such as in the foregoingFIGS. 18(a) and 18(b), the piezoelectric elements can be formed withoutfail proximately to maximum stress even if there is a slight amount ofmask slippage, and the conventional problem of an inability to detectmuch stress in a case whereby mask slippage has occurred can beeliminated.

Additionally, according to a knock sensor indicated in the first orthird embodiment, wherein a fixing pedestal 9 such as is indicated inFIG. 19 is fixed to the housing or to the connector by means of weldingor gluing or the like by means of a fixing portion indicated by 50,because the structure is such that the perimeter of the fixing pedestal9 is fixed, resonant frequency drops the most when a sensing element 11and signal processing circuits 10a and 10b and so on are mounted in acenter portion thereof. Consequently, if the center of gravity of theforegoing element and signal processing circuits and so on is made notto be placed on the centerpoint of gravity as seen from the fixingportion of the fixing pedestal, decline in resonant frequency can besuppressed to a certain extent. Additionally, as shown in FIG. 4, in adevice of a structure wherein a pedestal (substrate 17) further existsbetween the fixing pedestal 9 and the sensing element and signalprocessing circuit and the like, it is acceptable if the combined centerof gravity of the pedestal thereof and the sensing element and signalprocessing circuit and the like does not become the centerpoint ofgravity seen from the fixing portion. That is to say, it is acceptableif, at the centerpoint of gravity of the fixing pedestal whereat theperimeter portion thereof is fixed, the apparent centerpoint of gravityof the combined weight of elements mounted thereabove does not overlap.

INDUSTRIAL APPLICABILITY

The present invention, as a vibration detection device to detect aknocking phenomenon in an engine mounted in a vehicle or the like, canbe applied as a knock sensor while employing a structure wherein afabrication process is simple and moreover which can detect even up to ahigh-frequency region with no drop in sensitivity.

What is claimed is:
 1. A knock sensor, comprising:a housing installed onan engine; a connector portion assembled on said housing to form anisolated interior together with said housing, said connector portionincluding a lead electrically connecting between a side of said interiorand side of an exterior of said knock sensor; a pedestal disposed withinsaid isolated interior and having a resonant frequency of 40 kHz ormore; and a sensing means fixed to said pedestal and having a weight of1 g or less, said sensing means including a sensing element which has aframe part, a weight part set in said frame part and apart from saidframe part, and a plurality of beam parts connecting said weight partwith said frame part to doubly support said weight part within saidframe part, wherein said frame part, said weight part and said beamparts are formed of semiconductor substance, and said sensing elementincluding a detector detecting a vibration of said weight partresponsive to a knocking occurring in said engine, wherein each of saidbeam parts has a geometry selected so as to make a resonant frequency ofsaid sensing element 40 kHz or more to ensure that a maximum detectionfrequency of said detector is approximately 15 kHz.
 2. A knock sensoraccording to claim 1, wherein said frame part of said sensing elementhas a square configuration, said weight part has an oblong configurationand is disposed in said frame part to be positioned substantially at acenter of said frame part, and said beam parts are connected so as tosupport said weight part from two opposing side of said frame part andare disposed two by two from one side of said two opposing sides of saidframe part.
 3. A knock sensor according to claim 1, wherein said sensingmeans is fixed to said pedestal at a surface side opposite to a rearside which faces said connector portion in said isolated interior.
 4. Aknock sensor according to claim 3, wherein said pedestal is fixed tosaid connector portion at said rear side thereof.
 5. A knock sensoraccording to claim 4, wherein said sensing means is sealed in saidinterior from a remainder of said interior by a can and said pedestal.6. A knock sensor according to claim 5, wherein said remainder of saidinterior is filled with an absorbing material.
 7. A knock sensoraccording to claim 6, wherein said absorbing material is a silicone gel.8. A knock sensor according to claim 1, wherein said resonant frequencyof said sensing element is selected to be higher than said resonantfrequency of said pedestal.
 9. A knock sensor according to claim 8,wherein said resonant frequency of said sensing element is selected tobe 60 kHz or more.
 10. A knock sensor according to claim 1, wherein saidgeometry of each of said beam parts is selected to ensure that saidresonant frequency of said sensing element is 60 kHz or more.
 11. Aknock sensor according to claim 1, wherein said detector includes apiezoresistance effect element disposed at least on said beam part. 12.A knock sensor according to claim 11, wherein said geometry of each ofsaid beam parts is selected so as to make said resonant frequency ofsaid sensing element 40 kHz or more as well as to make a sensitivity ofsaid detector 12 μV/G or more.
 13. A knock sensor according to claim 12,wherein each length of said beam parts from said frame part to saidweight part is within a range of 0.05 mm to 0.215 mm.
 14. A knock sensoraccording to claim 12, wherein each length of said beam parts from saidframe part to said weight part is within a range of 0.05 mm to 0.1 mm.15. A knock sensor according to claim 1, wherein said sensing meansfixed to said pedestal further includes a signal processing element. 16.A knock sensor according to claim 15, wherein said signal processingelement includes an amplifier circuit amplifying an output signaldetected by said sensing element, a knock signal discriminating circuitdetermining an occurrence of said knocking based on an amplified signalfrom said amplifier circuit, and a power supply circuit supplying powerto said circuits.
 17. A knock sensor according to claim 15, wherein saidpedestal is fixed to an inner wall of said interior at a peripheralportion of said pedestal, disposed positions of said sensing element andsaid signal processing element on said pedestal being shifted from acenter of gravity of said pedestal.
 18. A knock sensor according toclaim 17, wherein an apparent centerpoint of gravity formed by saidsensing element and said signal processing element is shifted from saidcenter of gravity of said pedestal.
 19. A knock sensor according toclaim 1, wherein said weight part is suspended at two opposing sides ofsaid frame part by two pairs of two adjacent beam parts, every beam partbeing provided with at least one piezoresistance effect elementpositioned proximately to one of 1) a connecting point between saidframe part and said beam part and 2) a connection point between saidweight part and said beam part.
 20. A knock sensor according to claim19, wherein every beam part is provided with one piezoresistance effectelement, and wherein, in each of said two pairs of said two adjacentbeam parts, on one of said two adjacent beam parts is disposed apiezoresistance effect element at a first side proximate to saidconnection point between said frame part and said beam part while on theother of said two adjacent beam parts is disposed a piezoresistanceeffect element at a second side proximate to said connection pointbetween said weight part and said beam part.
 21. A knock sensoraccording to claim 20, wherein four said piezoresistance effect elementsform a Wheatstone bridge in which piezoresistance effect elementsdisposed on the respective beam parts at the same side of one of saidfirst side and said second side are disposed at diagonally oppositesides of said Wheatstone bridge.
 22. A knock sensor according to claim19, wherein said frame part and said weight part have a thicknessgreater than a thickness of said beam parts.
 23. A knock sensoraccording to claim 22, wherein a portion of one of said frame part andsaid weight part where said piezoresistance effect element isproximately disposed has a thickness the same as said thickness of saidbeam part continuously connected therefrom.
 24. A knock sensor accordingto claim 22, wherein both marginal portions of said frame part and saidweight part where said two pairs of said two adjacent beam parts connecttherebetween have a thickness the same as said thickness of said beamparts.
 25. A knock sensor, comprising:a housing installed on an engine;a connector portion assembled on said housing to form an isolatedinterior together with said housing, said connector portion including alead electrically connecting between a side of said interior and a sideof an exterior of said knock sensor; a sensor element located insidesaid interior, comprising a frame part, a weight part set in said framepart to be apart from said frame part, and a plurality of beam partsconnecting said weight part with said frame part to doubly support saidweight part within said frame part, wherein said frame part, said weightpart and said beam parts are formed of semiconductor substance; andpiezoresistance effect elements disposed on said beam parts,respectively, to detect a vibration of said weight part responsive to aknocking occurring in said engine, wherein each of said beam parts has ageometry selected so as to make a resonant frequency of said sensingelement 40 kHz or more to ensure that a maximum detection frequency ofsaid detector is approximately 15 kHz.
 26. A knock sensor according toclaim 25, wherein each of said beam parts has a geometry selected so asto make a resonant frequency of said sensing element 40 kHz or more aswell as to make a sensitivity of said piezoresistance effect elements 12μV/G or more.
 27. A knock sensor according to claim 25, wherein eachlength of said beam parts from said frame part to said weight part iswithin a range of 0.05 mm to 0.215 mm.
 28. A knock sensor according toclaim 25, wherein each length of said beam parts from said frame part tosaid weight part is within a range of 0.05 mm to 0.1 mm.
 29. A knocksensor according to claim 25, further comprising a pedestal for fixingsaid sensing element thereon, disposed within said isolated interior andhaving a resonant frequency of 40 kHz or more.
 30. A knock sensoraccording to claim 29, wherein said resonant frequency of said sensingelement is selected to be higher than said resonant frequency of saidpedestal.
 31. A knock sensor according to claim 30, wherein saidresonant frequency of said sensing element is selected to be 60 kHz ormore.